You could boot an alternate medium, e.g. a LiveCD. Open up a shell and mount the root FS. Edit /etc/shadow and remove the password. Save, sync, reboot. There shouldn't be a root password now. Alternatively, boot as you would normally and exec: "sudo passwd root" (enter your sudo password/passphrase), then type new password for root without needing the old one. The latter assumes of course, that your normal user has sudo privileges as apropos.
